Blue Ocean Alliance Unveils Seawater-Lubricated Sterntube-Less Vessel Design

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Future newbuild ships of all types should be built without a stern tube and with a seawater-lubricated propeller shaft bearing arrangement, according to Blue Ocean Alliance’s Chris Leontopoulos, reports Marine Insight.

Blue Ocean Alliance

Speaking at a recent maritime industry forum in Hamburg, Mr. Leontopoulos said the stern tubeless ship design – jointly developed by Blue Ocean Alliance members ABS, Thordon Bearings, Shanghai Merchant Ship Design and Research Institute (SDARI), Wärtsilä, and the National Technical University of Athens (NTUA) – is so commercially, operationally and environmentally attractive that “I hope that in the future all ships are built like this”.

Kick-starting his presentation with the startling statistic that more than eight million liters of stern tube lubricating oil is polluting the oceans annually, Mr. Leontopoulos, ABS’ Vice President, Technology, EMEA, said the design interventions proposed by the group can deliver a commercial and environmental optimal vessel capable of saving hundreds of thousands of dollars in operational costs. This is without changing hull lines or existing class rules and regulations.

Mr. Leontopoulos went on to say that the use of a seawater-lubricated bearing with a tapered key design – the principal component that makes the stern tubeless ship possible – also removes the need for an aft seal and a shaft coating system.

The stern tubeless design features a seawater-lubricated bearing, a Wärtsilä forward shaft seal, a Water Quality Package, and a bulkhead seal.

The technical differences were immediately obvious when a graphic was shown comparing a ship with and without a stern tube.

Essentially, the design presents a dry internal inspection chamber aft of the engine room, in the space normally given to the stern tube.

This new space, big enough for engineers to stand up in to maintain or replace propeller shaft components while the ship is afloat, has been created by trimming stiffeners and cutting an opening into the bulkhead.

The stern tubeless ship, he said, also negates the need to realign the shaft throughout the vessel’s lifetime, mitigating the risk of shaft damage or indentation, which can result in substantial downtime and expenditure.

Going on to reveal the cost benefits of a stern tubeless ship, Mr. Leontopoulos acknowledged that although the estimated CAPEX for additional components could add about US$10,000 to $30,000 to the cost of a newbuild depending on shaft diameter, the ROI is quick given the reduced operational expenditure. This is where the real financial gain is to be made.

He told conference delegates that aside from immediate savings of US$350,000 in lubricating oil, operators could save more than US$1 million in through-life operational costs.

This is based on reduced fuel consumption, reduced drydocking and maintenance costs, and associated loss of earnings; shaft alignment optimization, lower bearing wear rates, better EEDI; and an increase in cargo carrying capacity.

The arrangement also reduces to zero the risk of environmental fines related to stern tube oil pollution.

On that latter point, ABS has now published a Requirements for Sterntubeless Vessels with Water-Lubricated Bearings guide, bringing together in one document all existing applicable rules for the design.

ABS is also introducing a new notation for the stern tubeless ship design, for which the classification society awarded an AiP in June 2022.

Chris Leontopoulos believes the first stern tubeless ship with a Thordon seawater-lubricated propeller shaft bearing system, probably based on an SDARI design, will be ordered within the next twelve months.

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Source: Marine Insight